Businesses, governmental organizations and other entities are increasingly saving large volumes of data necessary for daily operations. In order to provide proper protection of data for business and legal purposes (e.g., to ensure quick recovery of data in the event of a disaster or to comply with document retention requirements), entities often back up data to a physical media, such as magnetic tapes, on a regular basis. Traditional backup systems placed an application server, backup server, source device, destination device and a local area network (“LAN”) in the data path of backup operations. Under these systems, the LANs were becoming overburdened by the amount of data being copied. Often, the backup window (the period in which data unavailable for normal operations in order to permit backup) was too short to achieve a complete backup of data. Many entities have now implemented Storage Area Networks (“SAN”) to relieve much of the burden of mass data storage and backup from the LAN, freeing the LAN for more immediate data storage and manipulation operations.
The T10/99-143r1, “Working Draft SCSI Extended Copy Command” (the “99-143r1 Draft”) and NCITS T10 SPC-2 (SCSI Primary Commands-2) (“SPC-2”) provide a mechanism for a computer backup application to delegate actual data movement to third party devices known as “copy manager devices” or “data mover devices”. In addition to the extended copy commands, the backup application may issue media commands. For example, if either the source or destination of the extended copy command is a removable media device, then the backup application may first have to issue media commands to move the media to the proper position (e.g., to rewind the tape).
The copy manager devices move data from source devices to destination devices as designated by the backup application in “segment descriptors” which in part constitute the parameter list of an extended copy command. The copy manager devices translate the extended copy commands into a series of lower-level read/write commands and execute the lower-level commands. The extended copy command is often used to backup data from random access devices, such as hard drives, to sequential access devices, such as tape drives.
The execution of an extended copy command to backup data carried out over a SAN may fail because of problems with tapes, tape drives, tape libraries, and the transmission of data across the SAN. An attempt to recover from the failure may fail, because the data is no longer available. The data, for example, may have been modified before the recovery attempt, and no other copy of the data may be available. To recover from backup failures, some entities use disk to disk to tape backup mechanisms. Backup is first performed from primary disk storage to secondary disk storage and then from secondary disk storage to tape. The data in the secondary disk storage is preserved, at least until backup is successful. On the failure of an attempt to backup data from secondary disk storage to tape, the data is available unchanged for retrying the backup of the data. The use of disk to disk to tape backup mechanisms to provide guaranteed backup is relatively slow and inefficient in the use of space. The copying of data from primary disk storage to secondary disk storage adds considerably to the time required for backup. In addition, as successive backups are made, the copying of data from primary storage to secondary storage requires space for multiple copies of the data copied from primary disk storage. Further, restoration is a cumbersome process, requiring the copying of data from the tape to the secondary disk storage followed by the copying of the data from the secondary disk storage to the primary disk storage.